Enormous bubbles observed on the surface of a red giant

The Sun’s surface is speckled with roughly two million convective cells (imagine boiling water), each about 1,000 miles (1,609 kilometers) across. Though the Sun’s pockmarks – more scientifically known as granulation patterns – have been well-studied over the years, never before have astronomers observed the convective cells of another star.

You guessed it: until now.

For the first time, astronomers using the PIONIER instrument on the European Southern Observatory’s (ESO’s) Very Large Telescope have directly observed the surface activity of another star. Unsurprisingly, the star is giant; a cool red giant to be precise.

Located about 530 light-years away in the constellation Grus (the Crane), the red giant in question, π¹ Gruis, is about the same mass as the Sun, yet its radius is 700 times as large. It also shines several thousand times brighter than the Sun.

In the study, published December 20 in the journal Nature, an international team of astronomers led by Claudia Paladini of the ESO described how the surface of π¹ Gruis contains only a few convective cells. However, these cells are enormous, with each one spanning around 74.5 million miles (120 million km). That works out to be about a quarter of the red giant’s diameter.

In roughly five billion years, the Sun will convert the last of the hydrogen in its core to helium. When this happens, there will be no more thermonuclear explosions blasting our star outward from within. The Sun will begin to collapse under its own gravity without hindrance. As it does, the pressure inside the Sun will increase, reheating our star by igniting a shell of hydrogen around its now-helium core. At this point, the Sun will have reached the red giant phase of its life.

π¹ Gruis is much further along than this in its red giant phase. It has moved on from burning its hydrogen shell, and is now countering gravity by fusing helium into heavier elements like carbon and oxygen.

In fact, π¹ Gruis is one of the brightest members of a relatively rare group of stars known as S-class stars, so-called because fusion in these stars follows the slow neutron capture process (s-process). This process is extremely important as it is responsible for the production of nearly half of the elements heavier than iron.

Because π¹ Gruis is an aging red giant, it has already ballooned outward to hundreds of times its original size. Over time, the star’s internal pressure will overpower the gravity holding onto its outer shells. According to the ESO’s press release, “While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like this one gradually expel their outer layers, resulting in beautiful planetary nebulae.”

This transition is likely already underway. Previous observations of π¹ Gruis have revealed it is surrounded by a shell of material almost a light-year from its surface. This cooling shell of gas is believed to have been ejected from the star around 20,000 years ago.

“This relatively short period in a star’s life lasts just a few tens of thousands of years – compared to the overall lifetime of several billion,” the press release stated. “And these observations reveal a new method for probing this fleeting red giant phase.”

While this may be the first surface of another star we have ever observed in such detail, it is certain to not be the last.